The wipe is set to correspond with the release of a major patch to Hearthstone: Heroes of Warcraft. What that patch may entail is currently unknown, but I think it's safe to assume it will contain some rebalancing of certain known, imbalanced cards. Beyond that, it's anybody's guess.
If you have purchased cards in the Hearthstone beta, or entered the arena, you will be reimbursed your purchase with in-game gold. Each pack is worth 100 gold, and each arena entry is worth 150 gold. Thus, if you bought the two-pack option for $2.99, you will receive 200 gold for that purchase, and if you bought, say, three arena entries, you will receive 450 gold for those entries. If you earned the Gelbin Mekkatorque legendary, you will receive him again after clearing the tutorials.
There's something I'm forgetting. What is it? Oh, yes. Any cards or rankings you earn after the closed beta wipe, you get to keep. Forever. Into the actual game's release. Don't believe me? Check out the official post for more information!